In response to excitation by sunlight, chlorophyll in plants fluoresces at wavelengths from about 660 nm to 800 nm. This behavior can be exploited for remote monitoring of the health of plants. For example, the fluorescence intensity from a plant increases when the plant lacks adequate water. Thus, fluorescence intensity data is useful, for instance, in making decisions concerning allocation of irrigation resources.
Discrimination between a signal emitted by a sunlit target and sunlight of the same wavelengths scattered from the target surface is one difficulty in detecting fluorescence from plants. One approach to differentiating the plant signal is spectral line discrimination. A spectral line discriminator observes light from a narrow range of wavelengths in which one or more absorption lines exist in the solar spectrum. (These absorption lines in the solar spectrum are known as Fraunhofer lines, and thus the terms "spectral line discriminator" and "Fraunhofer line discriminator" are used interchangeably in the literature in this field. The lines may result from either atomic species in the sun's atmosphere or from molecular species in the earth's atmosphere.) Specifically, if such a line is fully opaque, that is, having negligible spectral intensity of sunlight at its center, the signal observed at the line center consists entirely of fluorescence from the target, even if at nearby wavelengths outside the absorption line the intensity of the scattered sunlight is much larger than that of the fluorescence.
One such spectral line discriminator detects signals at the center of the line at 656.3 nm due to absorption by atomic hydrogen in the solar atmosphere. This approach is deficient in three respects. First, the intensity of the chlorophyll fluorescence at 656.3 nm is very weak. Second, the absorption line is not fully opaque at its center. These two factors in combination limit the sensitivity attainable with this method. Finally, the narrowband filter required to observe the line center is realized as a Fabry-Perot cavity. This is an expensive component requiring great precision in manufacture.